Harvesting energy in every possible environment requires innovative harvesting methods and circuit interfaces. For example, sensors can analyze a bridge's structural integrity. However, underneath a large suspension bridge battery replacement proves difficult and solar power can be unavailable. A combination of radio frequency (RF), thermal, and vibration harvesting can power these sensors in every location. The type of harvester determines the methods used in the interface circuit. A thermal harvester's DC output allows DC-DC start-up techniques not possible with an AC output. Similarly, an RF harvester's high frequency (˜MHz) very low power (˜nW) AC output can allow techniques not possible with a vibration harvester's low frequency (<500 Hz) low power (>1 μW) output. Even piezoelectric vibration harvester's capacitive element motivates different interface circuits vs. electromagnetic harvesters.
While these harvester outputs are different, they present similar challenges. One low voltage low power AC or DC harvester output must frequently cold start-up the interface circuit. The harvester must also supply nA-A of current to a load at a specific voltage. This may require boosting and must be done without transformers or a pre-charged supply. For example, a Cockcroft-Walton (CW) charge pump allows frequent start-up. This charge pump has an AC input and a DC reference. The charge pump boosts from the DC reference voltage to an output storage voltage (VStore). The switches that boost the charge pump can be passive diodes in either direction or active NMOS or PMOS diodes/switches. The active switches would need a DC power supply to generate a switching signal at their gates. Both MOSFETS and diodes have a technology dependent voltage that must be overcome to allow significant charge flow. In MOSFETs, this is called the threshold voltage (Vth). The transistor and diode threshold voltages limit the minimum input voltage required to turn-on the harvester. This disclosure focuses on reducing the harvester turn-on voltage.
Advances in wireless sensors and microprocessors have reduced power and voltage requirements. Some sensors require <6 nWs; however, low-power microprocessors may still require ˜300 μW at ˜1.8 V. These requirements determine the harvesting method and storage requirements. For example, an on-chip battery or capacitor may be sufficient to supply ˜nW sensors, but a microprocessor requires off-chip storage (e.g. a ˜100 μF capacitor). External storage is necessary because the harvested output can be and power limited (˜5-10 μW); nevertheless the power supply must be stable. The proposed charge pump could be part of a wireless multi-harvester system used to supply a microprocessor regularly in sleep mode. Here an IC allows the use of design techniques and technology characteristics, unavailable in discrete electronics to meet a system's requirements.
RF and vibration harvester circuits are both designed to overcome diode drops in rectifiers and charge pumps. However, low voltage start-up (˜200 mV) remains a challenge. High frequency (˜Mhz) very low power (˜nA) RF harvester outputs can readily be boosted and rectified as seen in FIG. 1A. This is because the diode turn-on voltage reduces by ˜100 mV for every decade of current from ˜μA to ˜nA. In fact, analysis shows that a diode's turn-on plays no role when harvesting deep sub-threshold current for a ˜nF load. Sensitivity in dBm can be used as a metric for RF circuits instead of efficiency. A charge pump and ˜nF-˜pF capacitive load is the primary option for high frequency very low power RF inputs.
Larger capacitive (˜μFs) loads will not charge from ˜nA current due to high leakage. Therefore, turn-on voltages remain an obstacle when charging >1 μF loads in RF harvesting. Zero-threshold diodes have limited effectiveness due to leakage. Floating gate capacitors reduce turn-on voltages when using a 10 μF load. However, the charge on these floating gate capacitors cannot be maintained for long-term bridge harvesting motivating new solutions.
Higher power DC-DC harvesting circuit research often focuses on power tracking, multi-harvester systems, or both. In thermal harvesting circuits, boosting is often required, but rectification is not required as seen in FIG. 1B. In most cases, to boost a low voltage DC output, it is necessary to generate a clock. The power source for this clock can be either an external source or the DC harvester input. This clock is then used to switch the MOSFETS in a charge pump or to provide a clock with the correct duty cycle for an LC boost converter. Both architectures can also be used together. Low-voltage duty cycle and LC converter control is difficult, so alternative start-up methods are required.
These alternative methods include the use of a transformer, external switch, pre-charged supply, external clock, sub-Hz trigger, or charge pump. On-chip charge pumps can boost using a clock created from a low DC voltage (˜95-330 mV). Similar to RF research, these charge pump thermal harvesting circuits start-up charging ˜pF-nF capacitors to reduce a switch's turn-on voltage. After >0.1 s with a DC input, this circuit transitions to a high current load, such as a 100 μF capacitor or small resistor. For a ˜100 Hz AC input, where rectification is required, this method for turn-on voltage reduction in a switch or diode is not an option to charge >1 μF loads from start-up.
The type of vibration harvester (e.g. piezoelectric vs. electromagnetic) determines the interface circuit architecture and challenges. Piezoelectric harvesters have a capacitive element that can be manipulated for higher efficiency. They tend to have a higher voltage output and use a buck converter. Charge pumps can also be used with low voltage piezoelectric harvesters. An inductor in these charge pumps optimizes matching with the piezoelectric harvester and its capacitive element.
Broadband vibration harvesters capture energy from a range of frequencies. One approach is to simultaneously rectify multiple frequencies from a piezoelectric disk. Another approach is to tune a harvester's resonant frequency, but this consumes too much power. An electromagnetic parametric frequency increased generator (PFIG) can harvest broadband frequencies and accelerations on a bridge (e.g. 30-100 mg (1 g=9.81 m/s2) between 1-30 Hz). It uses mechanical frequency up-conversion. A large mass snaps back and forth between latching magnets on two springs. When the mass is released, each springs oscillates at an up-converted frequency. A power generation magnet on top of each spring produces a decaying voltage when it oscillates between a copper coil. The decaying sine-wave voltage oscillating at ˜100 Hz has peak open circuit outputs decreasing from ˜450 to ˜200 mV with a 200-300Ω output impedance. In a year-long bridge study, a single FIG output regularly exceeded 10 μW for ˜20 seconds. Nights and weekends generated little power, emphasizing the need for circuit start-up.
Half-wave CW charge pumps can be used as building blocks to boost an interface circuit's output. The charge pump both boosts and rectifies as seen in FIG. 1C. Previous PFIG interface circuits used half-wave CW charge pumps made from off-chip capacitors (>1 μF) with and without transformers. Charge pumps enable boosting and start-up as the input is harvested. At the same time, the input is used as a clock to switch the diodes. A transformer can be added between a harvester and a charge pump as seen in FIG. 2A. The transformer boosts the harvester's output to overcome the charge pump's diode drops. Two half-wave CW charge pumps, each with its own input, can be added to produce a high voltage by connecting one charge pump's output to the next charge pump's DC reference as seen in FIG. 2B. A half-wave CW charge pump only harvests either positive or negative voltages depending on the diodes' orientation. However, a full-wave charge pump, with only one input, can be created with two half-wave CW charge pumps as seen in FIG. 2C. The two charge pumps' DC references are connected together, and one charge pump harvests positive voltages, while the other harvests negative voltages. A common ground for a multi-harvester system can be used by connecting the bottom charge pump output to ground.
The PFIG's passive charge pumps used Schottky diodes with a ˜180 mV turn-on voltage, Vdiode-drop. The maximum voltage, VoutCW, of a single n-stage CW charge pump to rectify and boost a decaying sine-wave with peak voltage, Vpeak, isV_outCW=2×n×(V_Peak−V_(diode-drop).  (1)With a 180 mV Vdiode-drop, the PFIG's low voltage ˜220 mV output gives a maximum circuit output voltage of <0.5 V (6 stages), with low efficiency (5-10%). Schottky diodes have also been used to create a supply with a dual output harvester. One harvester output creates a supply while the other output is actively harvested. However, this means only one of two outputs is harvested.
Transformers boost a harvester's AC signal to overcome diode drops. For a transformer matched to the harvester's impedance, the maximum charge pump output isV_CW=2×n×(T_Boost/2×V_Peak−V_(diode-drop)).  (2)TBoost is the transformer's boost ratio. When matched to a low impedance (˜3-4Ω), electromagnetic interface circuits produce high efficiencies (˜65%). However, circuits that interface to a high impedance (˜300Ω) low frequency FIG output have reduced efficiency (˜38%). This is due, in part, to the matched transformer's increased DC resistance and decreased inductive coupling. Also, a harvester designed to match to a transformer may be power limited.
Several circuit techniques reduce turn-on voltage for vibration harvesters. A passive negative voltage converter (NVC), similar to a gate-cross-coupled rectifier, converts the negative portion of the input positive for use in a half-wave rectifier. Once the NVC's input overcomes the technology's Vth, its voltage drop is ˜0 V. However, start-up voltage is still Vth dependent (0.35 μm CMOS) at ˜380 mV. Similar to RF circuits, passive low voltage (˜150 mV) sub-threshold charging of on-chip ˜pF-nF capacitors is also possible with vibration harvesters. This storage is not suitable for many applications though.
Active AC-DC solutions to boost and rectify improve efficiency, but they have difficulty with cold start-up and low voltage operation similar to DC-DC LC boost converters. Active fully integrated switched capacitor/charge pump ICs have been built that boost vibration harvester outputs, but they need a 0.8 V pre-charged supply to generate a clock. Rectifier-free AC-DC LC solutions still need a pre-charged supply. AC-DC LC boost converters don't always need a clock or pre-charge supply for start-up, but they still need to rectify and this rectification still depends on the diode's turn-on voltage.
Passive charge pumps can start-up between 0.44-0.5 V. Single and multiple stage discrete active diode charge pumps have been built for vibration harvesters using 100-200 μF capacitors. The large capacitors are needed to optimize efficiency by not limiting current. The bulk-connections of the discrete transistors form passive diodes for start-up. Well characterized efficiencies exceed 70% for higher power inputs at <1 kHz. With a comparator for every active diode, power consumption is ˜6 μW.
This section provides background information related to the present disclosure which is not necessarily prior art.